Electrical speakers are used to convert electrical signals to acoustical signals. The speakers may vary greatly in size and power, but generally they can be divided into two categories: dynamic speakers such as cone speakers commonly used in home HI-FI sets, and electrostatic speakers such as piezoelectric speakers used in buzzers, for example in digital watches and electric alarm clocks. Dynamic speakers typically have an electromagnetic actuator that moves a vibrating element (typically a diaphragm) that passes its vibration to the surrounding air and causes audible acoustic signals.
U.S. Pat. No. 4,653,103 presents one dynamic speaker implementation in which a diaphragm carries a plurality of voice coils attached on both sides of the diaphragm. The diaphragm is sandwiched between two yoke plates, which bear columnar magnets. The yoke plates have holes for allowing the passage of sound. In this implementation, the actuator drives the diaphragm practically across all of its area.
Piezoelectric speakers belong to the category of electrostatic speakers. Piezoelectric speakers are based on a piezoelectrically bending plate or strip that vibrates when subjected to an alternating electrical current. Piezoelectric speakers do not require much space and they are light, but their frequency response is often worse than that of dynamic speakers, and as the maximum movement of their sound-producing element (the plate) is relatively short, compared to dynamic speakers, they are outperformed by dynamic speakers at lower frequencies.
In general, the smaller the speaker, the more limited are its power, frequency response linearity and frequency band of sound production. The acoustic power a speaker produces is a product of the area of the cone (within a plane transverse to the movement of the cone) and the length of the movement.
Speakers are used in mobile devices such as portable radios, mobile telephones, portable computers, Personal Digital Assistant (PDA) devices and electronic games. In many such mobile devices, their size (volume) determines the amount of functionality the devices can have, that is the amount of electronics and battery space that they can contain. Therefore, it is desirable to reduce the amount of space occupied by all components of such devices including the space occupied by speakers.
As mentioned in the foregoing, piezoelectric speakers are small and light, but they have limited audio quality, particularly since their frequency response is moderate at low acoustic frequencies. On the other hand, dynamic speakers typically have a construction based on a magnet sound coil pair in the middle of the speaker to actuate a vibrating diaphragm. The geometry of the speaker is important to avoid undesired effects well known in the art, such as narrow-band resonance and rocking.
FIG. 1 shows an example of a dynamic (descant) speaker 100 in a sectional view. The speaker has a conical frame 110, which co-axially supports an actuator 120 and compliant surround 112, aligning all these around a shared central axis. The surround 112 attaches a cone 130 to the frame 110 at a mouth of the cone 130 (that is, at the broader end of the cone 130). At the throat (the narrower end of the cone 130) the cone 130 is shaped so that it forms a short cylindrically shaped voice coil support or former 134. The former 134 may be a seamless extension of the cone 130 or a part fixed to the cone 130, but in either case it is here referred to as a part of the cone 130. In addition to the surround, a so-called spider 114 (also co-axial with the frame 110, actuator 120 and surround 112) supports the former 134 with respect to the frame 110. The cone 130 is thus supported such that it is allowed to move easily closer to and away from the actuator 120, but restrained from free movement in a transverse direction. The access of dust into the throat is blocked by a dust cap, which is mounted on the cone 130 and thus vibrates together with the cone 130 when the speaker 100 is used.
The actuator 120 comprises two main parts: a magnet 121 and a voice coil 127, which together convert an electrical signal into vibration. The voice coil 127 is fixed to the former 134 and the magnet 121 is fixed to the frame 110. The actuator further comprises a front plate 122, a back plate 123 and a pole piece 126, all of these being circular, for making a stronger and more homogenous magnetic field through the voice coil 127. The voice coil 127 on the former 134 is surrounded by the front plate 122 and the pole piece 126 such that a narrow air gap 125 is left between the front plate 122 and the pole piece for receiving the former 134 and the voice coil 127.
As is apparent from FIG. 1, the actuator 120 occupies the central area behind the cone 130 except for a vent 124 left in the centre of the pole piece 126. This reduces space available for locating any other components behind the voice coil 127. This is particularly inconvenient in relatively thin devices such as portable information processing devices, because the speaker 100 may easily consume most of the depth of such a device.
With plate speakers, as opposed to cone speakers, the vibrating surface may be manufactured to be thinner, but resonance tends to adversely affect the audio response selectively at the resonance frequency bands. WO 97/09840 discloses one alternative dynamic plate speaker, wherein a different approach has been taken to deal with the generally undesired resonance phenomena. There, a single dynamic actuator is placed underneath a stiff cover plate of a speaker box for vibrating the cover plate. The actuator is non-centrally positioned in relation to the area of the cover plate such that it causes the cover plate to resonate over a broad frequency band thus improving the efficiency of the speaker.
NXT™ has published a Distributed-Mode Loudspeaker (DML) integrated into a visual display device. This speaker has been called NXT SoundVu. Its operation is based on bending waves excited in a transparent cover placed in front of a display. Exciters located at the edges of the transparent cover excite it. The exciters are fixed to a frame surrounding and supporting the transparent cover. The speaker can be used in laptop computers, where the screen is large enough to allow bending the cover on different edges with different exciters according to different signals. The implementation of the SoundVu speaker in each particular use requires solving coupled acoustics problems involved. For designing different SoundVu speakers (with different display devices and transparent covers), the NXT™ have developed special software programs. With these programs computer manufacturers should be able to design custom SoundVu speakers.
The bending of the cover is advantageous in the sense that the air gap behind the cover can be shallow, for example 2.75 mm. This shallow a gap is possible because bending waves of the transparent cover do not move the entire transparent cover as a single vibrating element, thus differing from the plate speaker and the cone speaker described in the foregoing. The sound-production with bending waves is thus far less limited by atmospheric pressure than the sound-production with plate and cone speakers. The bending of a transparent cover can be used in a speaker with relatively large transparent covers, such as a cover for display of a laptop computer having a 35-cm (14-inch) diameter. With smaller displays the coupled acoustics problems becomes more difficult reducing the sound-production performance of the speaker.
It is an object of the invention to avoid or at least mitigate the problems of the prior art.